The teres major is a muscle located in the shoulder. It assists in movements of the arm. This muscle originates on the scapula and inserts into the humerus. Exercises that work it include rowing and pull-up activities. An injury of this muscle will cause pain in the back of the shoulder and the back of the upper arm. The specific arm movements enabled by the muscle include pulling the arm toward the body, rotating the arm inward toward the body, and extending the arm back behind the waist.
The function of the teres major muscle is to adduct, rotate, and extend the arm. In this context, the term "adduct" refers to the ability to pull the outreached arm down and toward the body. Rotation the arm internally or across the belly is another function of this muscle. For example, if the arm is positioned at the side of the body with the forearm raised to create a 90-degree angle, rotating the fist inward so that the forearm crosses the belly creates an internal rotation. Extension with the muscle is used in pulling the arm back behind the body, as one would do when clasping the hands behind the body or preparing to pull up the back of a pair of pants.
The points of origin for teres major are the lower lateral border and the inferior angle of the scapula bone. The course of the muscle runs from the scapula to back of the upper arm. The point of insertion is the medial lip of the bicipital groove of the humerus. Innervation of the muscle is through the lower subscapular nerve, which consists of fibers from the fifth and sixth cervical vertebrae. This nerve also supplies the subscapularis muscle, which is located in close proximity to teres major.
An injury of the teres major muscle causes pain in the back of the shoulder and upper arm. Activities that can aggravate an injury include reaching above the head and across the body, and reaching behind the waist to pull up a pair of pants. Exercises such as pull-ups, pull-downs, shoulder rotations, and rowing will work and stretch the muscle.
The common term "rotator cuff" refers to a group of muscles that allow rotation of the shoulder. It is a common misconception that the teres major muscle in included in this group. The teres major, however, is not part of the rotator cuff. The muscles that make up the rotator cuff are the supraspinatus, the infraspinatus, the subscapularis, and teres minor.